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ref: 5a48412fda3c7104c0a8c83bc3507de619a5ba9b
dir: /silk/float/silk_burg_modified_FLP.c/

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#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include "silk_SigProc_FLP.h"

#define MAX_FRAME_SIZE              384 // subfr_length * nb_subfr = ( 0.005 * 16000 + 16 ) * 4 = 384
#define MAX_NB_SUBFR                4

/* Compute reflection coefficients from input signal */
SKP_float silk_burg_modified_FLP(   /* O    returns residual energy                                         */
    SKP_float       A[],                /* O    prediction coefficients (length order)                          */
    const SKP_float x[],                /* I    input signal, length: nb_subfr*(D+L_sub)                        */
    const opus_int   subfr_length,       /* I    input signal subframe length (including D preceeding samples)   */
    const opus_int   nb_subfr,           /* I    number of subframes stacked in x                                */
    const SKP_float WhiteNoiseFrac,     /* I    fraction added to zero-lag autocorrelation                      */
    const opus_int   D                   /* I    order                                                           */
)
{
    opus_int         k, n, s;
    double          C0, num, nrg_f, nrg_b, rc, Atmp, tmp1, tmp2;
    const SKP_float *x_ptr;
    double          C_first_row[ SILK_MAX_ORDER_LPC ], C_last_row[ SILK_MAX_ORDER_LPC ];
    double          CAf[ SILK_MAX_ORDER_LPC + 1 ], CAb[ SILK_MAX_ORDER_LPC + 1 ];
    double          Af[ SILK_MAX_ORDER_LPC ];

    SKP_assert( subfr_length * nb_subfr <= MAX_FRAME_SIZE );
    SKP_assert( nb_subfr <= MAX_NB_SUBFR );

    /* Compute autocorrelations, added over subframes */
    C0 = silk_energy_FLP( x, nb_subfr * subfr_length );
    SKP_memset( C_first_row, 0, SILK_MAX_ORDER_LPC * sizeof( double ) );
    for( s = 0; s < nb_subfr; s++ ) {
        x_ptr = x + s * subfr_length;
        for( n = 1; n < D + 1; n++ ) {
            C_first_row[ n - 1 ] += silk_inner_product_FLP( x_ptr, x_ptr + n, subfr_length - n );
        }
    }
    SKP_memcpy( C_last_row, C_first_row, SILK_MAX_ORDER_LPC * sizeof( double ) );

    /* Initialize */
    CAb[ 0 ] = CAf[ 0 ] = C0 + WhiteNoiseFrac * C0 + 1e-9f;

    for( n = 0; n < D; n++ ) {
        /* Update first row of correlation matrix (without first element) */
        /* Update last row of correlation matrix (without last element, stored in reversed order) */
        /* Update C * Af */
        /* Update C * flipud(Af) (stored in reversed order) */
        for( s = 0; s < nb_subfr; s++ ) {
            x_ptr = x + s * subfr_length;
            tmp1 = x_ptr[ n ];
            tmp2 = x_ptr[ subfr_length - n - 1 ];
            for( k = 0; k < n; k++ ) {
                C_first_row[ k ] -= x_ptr[ n ] * x_ptr[ n - k - 1 ];
                C_last_row[ k ]  -= x_ptr[ subfr_length - n - 1 ] * x_ptr[ subfr_length - n + k ];
                Atmp = Af[ k ];
                tmp1 += x_ptr[ n - k - 1 ] * Atmp;
                tmp2 += x_ptr[ subfr_length - n + k ] * Atmp;
            }
            for( k = 0; k <= n; k++ ) {
                CAf[ k ] -= tmp1 * x_ptr[ n - k ];
                CAb[ k ] -= tmp2 * x_ptr[ subfr_length - n + k - 1 ];
            }
        }
        tmp1 = C_first_row[ n ];
        tmp2 = C_last_row[ n ];
        for( k = 0; k < n; k++ ) {
            Atmp = Af[ k ];
            tmp1 += C_last_row[ n - k - 1 ]  * Atmp;
            tmp2 += C_first_row[ n - k - 1 ] * Atmp;
        }
        CAf[ n + 1 ] = tmp1;
        CAb[ n + 1 ] = tmp2;

        /* Calculate nominator and denominator for the next order reflection (parcor) coefficient */
        num = CAb[ n + 1 ];
        nrg_b = CAb[ 0 ];
        nrg_f = CAf[ 0 ];
        for( k = 0; k < n; k++ ) {
            Atmp = Af[ k ];
            num   += CAb[ n - k ] * Atmp;
            nrg_b += CAb[ k + 1 ] * Atmp;
            nrg_f += CAf[ k + 1 ] * Atmp;
        }
        SKP_assert( nrg_f > 0.0 );
        SKP_assert( nrg_b > 0.0 );

        /* Calculate the next order reflection (parcor) coefficient */
        rc = -2.0 * num / ( nrg_f + nrg_b );
        SKP_assert( rc > -1.0 && rc < 1.0 );

        /* Update the AR coefficients */
        for( k = 0; k < (n + 1) >> 1; k++ ) {
            tmp1 = Af[ k ];
            tmp2 = Af[ n - k - 1 ];
            Af[ k ]         = tmp1 + rc * tmp2;
            Af[ n - k - 1 ] = tmp2 + rc * tmp1;
        }
        Af[ n ] = rc;

        /* Update C * Af and C * Ab */
        for( k = 0; k <= n + 1; k++ ) {
            tmp1 = CAf[ k ];
            CAf[ k ]          += rc * CAb[ n - k + 1 ];
            CAb[ n - k + 1  ] += rc * tmp1;
        }
    }

    /* Return residual energy */
    nrg_f = CAf[ 0 ];
    tmp1 = 1.0;
    for( k = 0; k < D; k++ ) {
        Atmp = Af[ k ];
        nrg_f += CAf[ k + 1 ] * Atmp;
        tmp1  += Atmp * Atmp;
        A[ k ] = (SKP_float)(-Atmp);
    }
    nrg_f -= WhiteNoiseFrac * C0 * tmp1;

    return (SKP_float)nrg_f;
}